This study was undertaken to characterize the ternary complex composed of actin, AMP-PNP and the subfragments of myosin, subfragment-one (S-1) or heavy meromyosin (HMM). The actin and S-1 bind independently to form a ternary complex with one S-1 molecule binding per F-actin monomer. The binding of actin to S-1 AMP-PNP is relatively strong at low salt, but becomes weaker with increasing ionic strength. The association constant for AMP-PNP to actin S-1 is 5.4 times 10 to the 4th power M minus 1, measured at several ionic strengths. In addition, this study showed that the association constant for the binding of actin to HMM times AMP-PNP was only twice as strong as that obtained with S-1, indicating weak binding of the second head of HMM.